"I think Chuck Schumer's endorsement is the kiss of death," said Dean, who is backing rival DNC candidate Pete Buttigieg, mayor of West Bend, Ind.

"Because I had to say no to Chuck Schumer when he demanded $5 million, because I said, 'Chuck, you're not getting it for the DSCC; I'm putting it into local races," Dean added, referring to the Democratic Senatorial Campaign Committee.

Dean endorsed Buttigieg on Wednesday. Ellison of Minnesota and former Labor Secretary Tom Perez are viewed as the frontrunners.

Schumer had backed Ellison in November, while Perez has picked up the support of former Vice President Joe Biden.

CBS News said the tight contest, which will be decided on Saturday, remains fluid.